Binding of Ca2+ to lipid membranes in the fluid and gel state

X. RAFFO; M. ANGELES FRIAS; DIAZ S.; E.E. TYMCZYSZYN; DISALVO E. A.

Rosario – Santa Fé

Congreso; XXXV REUNION ANUAL DE LA SOCIEDAD ARGENTINA DE BIOFÍSIC; 2006

Sociedad Argentina de Biofísica

The binding of Ca2+ to dimyristoyl (DMPC) and egg phosphatidylcholine (ePC) membranes was determined by measuring the changes in the zeta potential of MLV dispersions. It was observed that, in the case of saturated lipids, the zeta potential varied between negative to positive values. In contrast, in egg PC the zeta potential was always below cero. In both cases, the zeta potential vs Ca2+ concentration curves reached saturation. The fact that the Ca2+ binding decreases the surface potential to positive values denotes that Ca+2 is neutralized, at least partially, the phosphate group of the phospholipids. It appears that in DMPC the membrane surface has a higher affinity for Ca2+. Thus, the packing or the hydration of the interfacial groups would affect the Ca2+ binding. The effect of Ca2+ solution on the antisymetric vibrational mode of the phosphate was studied by FTIR spectroscopy. The results are compared with those obtained with negatively charged lipids such as DMPG from literature. Two possible interpretations can be derived. In one of them, Ca 2+ binds to the phosphate without displacing water. The second one is that the water displacement is compensated by the electrostatic interaction of Ca2+ with the phosphate, thus the position of the absorption bands of the PO groups is unchanged.